First to third conventional LDD type MOS transistors have been proposed. The first conventional LDD type MOS transistor includes a gate electrode for covering areas on a silicon oxide film, under which N.sup.- type source.drain regions are formed on a P type silicon substrate by a predetermined depth. The second conventional LDD type MOS transistor includes silicon nitride film spacers provided on sides of a gate electrode, wherein the film spacers cover areas on a silicon oxide film formed on a P type silicon substrate, under which N.sup.- type source.drain regions are formed on the P type silicon substrate by a predetermined depth. The third conventional LDD type MOS transistor includes a silicon oxide film for covering a gate electrode and non-doped polysilicon film spacers provided on sides of the silicon oxide film-covered gate electrode, wherein the film spacers cover areas on a P type silicon substrate, under which N.sup.- type source.drain regions are formed on the P type silicon substrate by a predetermined depth.
In the first to third conventional LDD type MOS transistors, an accumulation layer is easy to be formed on the N.sup.- type drain region, so that an electric resistance of the N.sup.- type drain region is decreased. The detail of the first to third conventional LDD type MOS transistors will be explained prior to the explanations of preferred embodiments according to the invention.
In the first conventional LDD type MOS transistor, however, there is a disadvantage in that a capacitance is increased between the gate electrode and each of the N.sup.- type source.drain regions.
In the second conventional LDD type MOS transistor, there is a disadvantage in that the silicon nitride film spacers have a number of trap levels to lower the reliability.
In addition, the third conventional LDD type MOS transistor has a disadvantage in that operation is unstable, because a potential of the non-doped polysilicon film spacers is under a floating state due to the structure in which the nondoped polysilicon film spacers are not connected to any electrode or any interconnection.